No abstract

Nitrogen is a well-known gamma-stabiliser in austenitic steels, also responsible for significant solid solution hardening of these materials. Yet, only few papers have studied its impact on austenitic high-entropy alloy (HEA) matrixes. This study focuses on a cobalt-free, non equimolar CrFeMnNi HEA doped with nitrogen. A series of alloys was cast under a nitriding atmosphere to promote nitrogen absorption into the liquid alloy. Study of as-cast alloys has shown nitrogen presence in solid solution up to 0.3 wt. % (1.2 at. %). Over the whole range of compositions, a linear increase of hardness (134 HV/wt. % of N) was measured as well as an expansion of the lattice parameter of a/a= 1.01 / wt. % N due to nitrogen addition in the interstitial sites of the lattice. Tests on forged and annealed samples showed that the increase of hardness with nitrogen addition is higher than in as-cast state (210 HV / wt. % of N) surely due to presence of other strengthening mechanisms. Tensile tests confirmed that the presence of dissolved nitrogen increases yield strength and ultimate strength and enhances strain-hardening, without any modification of ductility.

This paper studies the superplasticity of conventional sheets of Ti-1V-4Al-3Mo (α+β) alloy. The flow behavior was investigated in a temperature range of 775 °C-900 °C and a constant strain rate range of 2×10 −4-5×10 −3 s −1 via uniaxial tensile tests. The microstructure evolution during the superplastic deformation was analyzed. The results revealed that, the flow behavior of Ti-1V-4Al-3Mo (α+β) alloy is characterized by strain softening phenomena. The experimental stress-strain data were used to build a power law constitutive model. A processing map, which shows the safe and unsafe regions of deformation, was also constructed for the studied alloy. The optimal deformation regime was attained at a temperature of 875 °C and strain rate of 1×10 −3 s −1 which provided a β phase fraction of 52%. Equiaxed fine-grained α and β structure with size of 2-3 μm as well as dislocation activity inside the α-grains were identified in the optimum deformation regime.